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Introduction to Linux - A Hands on Guide

This guide was created as an overview of the Linux Operating System, geared toward new users as an exploration tour and getting started guide, with exercises at the end of each chapter.
For more advanced trainees it can be a desktop reference, and a collection of the base knowledge needed to proceed with system and network administration. This book contains many real life examples derived from the author's experience as a Linux system and network administrator, trainer and consultant. They hope these examples will help you to get a better understanding of the Linux system and that you feel encouraged to try out things on your own.

I had a WinXP box connected to the internet and use Internet Connection Sharing to make it act as gateway to my home LAN. All was fine, but exposing Windows to the outer world didn't feel so right. Now I tried to set up a debian (sid) box as the gateway instead.

So, I apt-get install ipmasq, and instantly have my debian box usable as gateway! However, I discovered that the boxen behind the debian gateway can't access some website, while other website can be viewed just fine.

example: mail,yahoo,com login,yahoo,com www,msn,com www,microsoft,com gave timeout error. The rest of the world, like slashdot or google or other random sites, seem fine (including www,hotmail,com www,yahoo,com).

Also, windowsupdate gave error 80072EE2 after the screen "Windows Update is looking for available update" and sign in to msn messenger failed on those boxen.

On debian box, all the above is working fine (minus windowsupdate), though. Switching back to Windows gateway and everything works just fine again.

Could anyone give me an idea how to investigate this problem? I would appreciate this very much!
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(I think) related packages' versions
kernel-image-2.6.8 (custom built)
ipmasq 4.0.1
iptables 1.2.11-8

It sounds like you need to configure your Windows box (behind the Debian gateway) to use your ISP's DNS servers. I don't know how to do this with Windows XP, but with Windows 98, you had to go into Network Properties (from Control Panel), then configure your TCP/IP properties. There was a dialog called "DNS Servers" or something, then you had to manually enter the ip addresses for your ISP's DNS servers.

Your network works with a Windows XP gateway probably because Windows does internet sharing through dhcp by default, whereas with Debian you have to explicitly enable that service (it's a security feature). Check out my .sig for more documentation, especially the Linux Gateway how-to.

It's probably the mtu value you need to change of the network card on the client and the one connected to the client on the server. Usually you can ping the website, but the browser doesn't display it. Changing the mtu to 1452 should fix this.

Thanks! The problem is indeed caused by MTU over PPPoE link. I found a good description of this problem (and solution) after reading your post. Here
http://www.linux.com/howtos/IP-Masquerade-HOWTO/mtu-issues.shtml